1. Field of the Invention
The present invention relates to phosphors, and in particular relates to white light illumination devices utilizing the same.
2. Description of the Related Art
In 1962, Nick Holonyak Jr. of General Electric Company developed the first commercial visible light emitting diode. In the 1970s, GaP-based and GaAsP-based LEDs were developed for emitting red light, yellow light, orange light, green light, and the likes for indication lamps or digital displays. In 1991, the InGaAlP-based LEDs were developed for LEDs that had high brightness. The LEDs with high brightness were applied in traffic lights, brake lights, taillights, and the likes. Still in 1991, Shuji Nakamura of Nichia Corporation invented GaN-based and InGaN-based LEDs for emitting blue-violet light. It was the first commercial blue-violet LED product available at the time, which was applied in the backlighting of keypads and full color billboards. In 1996, Nichia Corporation collocated LED chips with a yellow phosphor ((Y,Gd)3(Al,Ga)5O12:Ce3+, YAG) to for commercially available white light LEDs. However, because a part of the blue light from the LED chip needed to be mixed with the yellow light from the yellow phosphor to produce white light, the white LEDs had a very high color temperature; especially when driven by high current. Also, YAG phosphors had lower emission efficiency at higher temperatures.
In 1999, U.S. Pat. No. 5,998,925 (Shimizu et al.) disclosed a blue light chip collocated with phosphors to produce white light, wherein the phosphors contained garnet fluorescent material activated with cerium, which contained Y, Lu, Sc, La, Gd, Sm, or combinations thereof, and Al, Ga, In, or combinations thereof. The phosphors in Shimizu et al. had low thermal stability, and it was easy for the phosphors to thermally decay under high temperatures.
Accordingly, a novel phosphor formula is called for, for meeting the requirements of high thermal stability and high emission intensity.